Why Climate Change Ups the Odds of Fires, FloodsColorado's record-breaking flood was caused, in part, by a blocking pattern parked over western North America. That same pattern also led to extreme drought in the West, worsening California's Rim Fire. Rutgers atmospheric scientist Jennifer Francis talks about possible connections between climate change and severe events like these.

Colorado's record-breaking flood was caused, in part, by a blocking pattern parked over western North America. That same pattern also led to extreme drought in the West, worsening California's Rim Fire. Rutgers atmospheric scientist Jennifer Francis talks about possible connections between climate change and severe events like these.

This is SCIENCE FRIDAY. I'm John Dankosky. Ira Flatow is away. Last week's record-breaking rains in Colorado unleashed devastating floodwaters along the front range of the Rockies, what some are calling a thousand-year flood. It destroyed some 1,700 homes and damaged 16,000 more. The cost of the destruction: at least $2 billion. What set off these flash floods, though, and will this sort of extreme weather, huge rains following a severe drought, happen more as the planet warms?

Here to talk about it is Jennifer Francis, a research professor at the Institute of Marine and Coastal Sciences at Rutgers University. Welcome to SCIENCE FRIDAY, Dr. Francis.

JENNIFER FRANCIS: Thank you, John, nice to be here.

DANKOSKY: You can give us a call. Our number is 1-800-989-8255. That's 1-800-989-TALK. If you're on Twitter, you can tweet us your questions by writing the @ sign, followed by SciFri. You can also leave us a comment on ScienceFriday.com where you will find some links to our topic. So Jennifer Francis, tell us what sorts of weather patterns led to this flooding in Colorado. Was this a rare occurrence?

FRANCIS: Well, they, of course, have had floods in the Front Range before of this magnitude, but there are a couple things about this particular one that made it very unusual. For one thing, the floods that they usually have this time of year are typically more of the flash flood variety, whereas the one that occurred last week was a very long lived event for Colorado.

It spanned a few days. Normally, they just come and go. And the amounts of rain are just really staggering. I mean, usually, in the month of September, say, Boulder, Colorado, would typically get about two inches. They got nine inches in just one day. And over the whole week, it added up to about 17 inches. And this can be understood in the perspective of their typical amount of rain they get for the whole year, which is only 20 inches.

So here we are in mid-September and they've already got more rain than they've ever had. 2013 is already the wettest year ever and there's still three and a half months left to go.

DANKOSKY: So what accounts for the length of the storm? Why did it just last so long?

FRANCIS: Well, it was a very interesting weather pattern. There was a large swing in the jet stream northward up over the whole western half of North America. So there was this big swing northward in the jet stream and then, south of that, was what we call an upper level low. So the jet stream was bringing in moisture from the Pacific while the lower level winds were actually from the southeast bringing moisture in from the Gulf of Mexico.

And so we had moisture at two levels in the atmosphere, which made for a very, very large amount of water vapor available. So as that wind came in from the southeast at low levels, it hit the Rocky Mountains, was lifted by the mountains so it then cools and condenses and it wrings that moisture right out.

And because there was this large swing in the jet stream and this upper level low, this configuration we call a blocking pattern. And it's called the blocking pattern for the very good reason that it means that the weather patterns are stuck. The block means that it's blocking things from changing. And so we were in this pattern for several days and we've been seeing these sorts of blocking patterns quite a lot in the last couple years.

DANKOSKY: So what's happening to cause the jet stream to do this? Does it have to do with climate change?

FRANCIS: Well, it's a hard call. I mean, blocks are natural. We see them fairly regularly around the Northern Hemisphere, but we think there are reasons to expect these sorts of blocking patterns to occur more often. In fact, we are starting to see them occur more often. In the summertime - it depends on which season you're talking about. But in the summertime and in this period of years that we're in right now in particular, we've had a prolonged drought in the western half of North America.

And we think this is at least partly related to the fact that, of course, sea ice is disappearing, the Arctic is warming very rapidly, we're losing the snow cover much earlier on high latitude land areas, so in the far northern part of North America, and this causes the soil underneath to be able to dry out and warm up much faster in the spring, which kind of gives a jumpstart to the warming season of summer.

So we think this might be contributing to the jet stream taking these very large northward swings because that soil, once it dries out, is able to heat up much more and it kind of creates a bulge or a dome of hot air. And this sort of contributes to the jet stream taking these sorts of patterns.

DANKOSKY: Because I've heard these patterns in the jet stream both described as slowing down the jet stream, but also taking this wavy, almost lazy path. Is that what is happening, it's essentially slowing down and getting more unusual looking from what we're used to seeing the jet stream do?

FRANCIS: Right. So that's the idea. And it's because the Arctic is warming much faster than the areas farther south and it's that temperature difference between the Arctic and areas farther south that really is what drives the west to east winds in the jet stream. So we are, in fact, seeing those west to east winds of the jet stream slowing down and we believe it's related to this differential warming of the Arctic.

And once the jet stream does slow down, it tends to take a wavier path as it goes around the Northern Hemisphere so bigger swings to the north and south, which, you know, we're starting to see that happening more.

DANKOSKY: So are there certain parts of the world that are going to be more affected by this sort of pattern than others? I mean, we see what's happening in the west, these very severe droughts, these very, very heavy rains. Are there some places that are going to be more heavily affected than others?

FRANCIS: Well, I think there's still a lot of research going on on this very topic because while we think the jet stream will take a wavier path, it's very difficult to predict exactly where those waves are going to set up, where the bulges northward are going to be and where the dips southward are going to be.

And it's those waves in the jet stream that create the weather that we feel down here on the surface. So where you're located relative to different parts of these waves determines what kind of weather you're in. But what we are seeing is an increasing frequency of these northward bulges, which we call ridges, occurring in the North Atlantic, in particular.

You can think about Sandy, for example, when Sandy came along, there was one of these very strong ridges or blocking patterns in the North Atlantic and that's what helped steer her into the East Coast. And we also see them becoming more frequent in the western U.S. and other areas as well. So it does seem that we're starting to see some patterns emerge that will help us make longer term predictions in the future.

DANKOSKY: We have a caller on the line. Kurt(ph) is calling from Boulder, Colorado. Hi, Kurt, go ahead. You're on SCIENCE FRIDAY.

KURT: Well, we dodged a bullet. I'm a graduate with Colorado School of Mines graduate school, and been doing mine reclamation work and I'm now living in an apartment building that straddles one of the ditches that's a farmers ditch. And I did a flood assessment of my own last June and figured out that our building was safe because the ditch is going to jump its banks before it threatens our building.

And, in fact, it did jump the banks and one of the neighboring buildings got 32 inches of water. But the thing is that the soil here was becoming saturated on Thursday. You know, we've been having such dry conditions that this rain was wonderful in the regard that it was coming in very slowly and gently, but when the soil saturated, you know, all the absorbness(ph) capacity was going to be gone and the rain quit just about exactly the time...

DANKOSKY: My goodness.

KURT: ...that, yeah, the whole town would've become just a, you know, a water producing thing instead of water absorbing some.

DANKOSKY: Well, I'm glad, Kurt, that it wasn't worse than it was. Thank you so much for your phone call. And before I let you go, Professor Francis, Kurt is saying they dodged a bullet. Do you think he did?

FRANCIS: Well, I'm not sure that everyone in that area would agree. I think they - some people really got hit by the bullet. But he is correct that, you know, there's been a prolonged drought in that area. So, actually, the soils were very dried out. And - initially, anyway - when the heavy rains came, a lot of it just ran right off, because the soil was so hard and packed and dry.

But then, you know, there's this period when the water really is soaking into the soil, which is a good thing. But then once they get saturated, then they just can't hold anymore, and so all of it runs off again. So I think there are areas where they're very happy for this rain. But, of course, it's too much of a good thing in some places, too.

DANKOSKY: And, of course, all the best to all the people of Colorado, who will be cleaning up for a very long time. Jennifer Francis is a research professor at the Institute of Marine and Coastal Sciences at Rutgers University. Thank you so much for joining us today.

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